Theory of Operation 5
Keysight 34970A/34972A Service Guide 139
Input Amplifier
Unless otherwise noted, components in this discussion are located on the A4
circuit assembly (34970-66504).
The DC Amplifier circuit is used by every measuring function except frequency and
period. Analog switch U101B selects various input signals for measurement by the
ADC. Switch U101B has three sources which can be dynamically selected:
measure customer input (MC), measure zero input (MZ), and precharge (PRE). The
MC state is the actual input measurement. The MZ state measures internal offset
voltages which are also present in the MC measurement. The final measurement
result is computed from MC–MZ. The PRE state is used to “precharge” internal
capacitances to reduce charge injection to the input terminal from the dynamic
switching of MC and MZ. Autozero off disables the dynamic switching of the
amplifier input. However, a new MZ value is automatically taken whenever a new
function or range is selected, even if autozero is turned off.
In the DC voltage function, ranging is accomplished through both input relay
switching (K102–K104) and solid state switching (U101). As a result, the input to
the ADC has the same nominal 10 V value for a full scale input on each range. The
DC input amplifier is comprised of source follower dual FET U104, amplifier U106,
and associated bias circuitry. The feedback resistors U102C and switches U101C
select non-inverting amplifier gains of x1, x10, and x100 for the DC input amplifier
circuit. Amplifier output ADIN drives the DC input to the a-to-d converter for all
measuring functions.
DCV Range U102 Divider U101 Input Amplifier Gain ADC Input
100 mV
1 V
10 V
100 V
1000V
1/100
1/100
Pin 5
Pin 5
Pin 5
Pin 8
Pin 8
x100
x10
x1
x10
x1
10 V
10 V
10 V
10 V
10 V
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